1. Related Field
The present invention concerns a control rod for a nuclear power light water reactor. The invention also concerns the use of such a control rod.
2. Related Art
In nuclear power light water reactors water is used as neutron moderator. There are two kinds of nuclear power light water reactors, i.e. boiling water reactors, BWR, and pressurized water reactors, PWR. In both kinds of light water reactors some kind of control rods are used for controlling or stopping the nuclear reaction. Such control rods contain a neutron absorbing material. A common neutron absorbing material is boron carbide, B4C. This material is mentioned for example in WO00/002205 A1. Another material that can be used is pure hafnium or hafnium with small additives of other materials. This is described for example in U.S. Pat. No. 5,330,589. It is also known to use control rods that comprise large plates of hafnium. Such control rods usually contain channels for water. For example U.S. Pat. No. 6,137,854 describes control rods with hafnium plates.
The article “Development of Advanced Control Rod of Hafnium Hydride for Fast Reactors” by Konashi at al. in Proceedings of ICAPP '06 Reno, Nev. USA, 4-8 Jun., 2006, pages 2213-2217 does not concern light water reactors but a completely different kind of nuclear power reactors, i.e. so called fast reactors. In a fast reactor the fission reaction is maintained with the help of fast neutrons. Such a reactor does normally not need any neutron moderator. Therefore there is in a fast reactor no water that functions as a neutron moderator. Also the nuclear fuel material that is used in a fast reactor differs from that which is used in a light water reactor. The mentioned article by Konashi et al. describes problems that may arise when using B4C as absorber material in a control rod in a fast reactor. The article describes that when boron carbide is used, helium gas may be generated, which may lead to the fact that the absorber material swells, which may cause damage on the material that surrounds the absorber material. In order to avoid this problem it is in the article discussed to instead use hafnium as absorber material. It is noted on page 2214 in the article that hafnium has a good absorption capacity for thermal neutrons and that hafnium therefore has been used in light water reactors. The article mentions (page 2214) that there is no experience from using hafnium as absorber material in a fast reactor since the neutron capture capacity of hafnium in a fast reactor is low in comparison with in a light water reactor. The article suggests the use of hafnium hydride as absorber material in a fast reactor since the hafnium hydride has a better neutron absorption capacity than hafnium in a fast reactor since the hafnium hydride also functions as a neutron moderator, i.e. the fast neutrons are slowed down and can therefore be better absorbed.